The technique of obtaining a high-strength, transparent molded article by mixing an isocyanate terminal prepolymer component (A) and an aromatic diamine component (B) mentioned above is described in U.S. Pat. No. 6,127,505.
The reaction of components (A) and (B) is one of the fastest among many molding reactions of urethane and derivatives thereof. Accordingly, a reaction injection molding (“RIM” hereinafter) machine is generally employed. In the mixing step, two solutions are thoroughly mixed in a mixer. They are then immediately cast into a casting mold, where they are cast-polymerized to obtain a molded article.
The configuration of a common RIM machine is shown in FIG. 18. The method of manufacturing a molded article using this machine will be described.
Here, components (A) and (B) are stored under a reduced pressure in material tanks 11A and 11B and thoroughly degassed. When degassing is inadequate, bubbles mix into the molded article, sometimes compromising the properties and appearance of the final product and sometimes reducing the mechanical strength of the molded article. Further, since component (A) is of high viscosity at an ordinary temperature, heating is used to reduce this viscosity when the efficiency of degassing is affected or molding is precluded. The two liquids are mixed and discharged once materials in the tanks have been thoroughly degassed and rendered uniform in temperature. In this process, the materials are forced from the tank through 13A and 13B by pumps 12A and 12B, pass through filters 14A and 14B, and arrive in mixing and discharging part. In mixing and discharging part 15, components (A) and (B) are mixed either by shafts rotating at high speed or by a static mixer to obtain a uniformly mixed solution of the two liquids, and the mixed solution is discharged.
Components (A) and (B) react very rapidly, requiring only several tens of seconds to several minutes to form a gel. Thus, when foreign matter is present in one or both components, it is extremely difficult to use a filter device to remove the foreign matter in a post-mixing step. This is not only because the filter device solidifies and ceases to function, but also because the control function of the mold pump is affected. Accordingly, when removing foreign matter from the components, it is necessary to position filter devices between mixing and discharging part 15 and pumps —12A and 12B. Further, to maintain a constant mixing ratio of components (A) and (B) and achieve molded articles of constant quality, the level (rate) at which components (A) and (B) are discharged to mixing and discharging part 15 must be stabilized to the extent possible. Thus, it suffices to position the filter devices as far as possible from mixing and discharging part 15. This permits stabilization of the unstable flow of the individual components following mixing.
For such reasons, filters 14A and 14B have been conventionally mounted near the positions indicated in FIG. 18 within the RIM machine, as stated above.
Filters having an absolute filtration precision of about 100 μm have generally been mounted in the RIM machine. However, when manufacturing optical members such as eyewear lenses, this level of filtration precision does not permit good removal of the minute foreign matter that tends to reduce molded article transmittance, optical properties, and mechanical strength.
The minute foreign matter causing these problems is comprised particularly of impurities of 10 μm or less. These impurities are difficult to remove without precision filtration with a filter of an absolute filtration precision of less than or equal to 10 μm.
Accordingly, it is conceivable to increase the filtration precision of the filter provided in the RIM machine. However, the filtering capability of the filter provided in the RIM machine greatly affects the discharge level of each component. Thus, when the filtration precision of the filter becomes excessively high, it may be impossible for the discharge level of each component to achieve the targeted discharge level. In particular, component (A) is of high viscosity and is difficult to filter at an ordinary temperature, and must therefore be heated to lower its viscosity. However, deterioration of component (A) due to heating, the heat resistance of the casting mold, and properties relating to ease of handling by the operator in the step of filling the casting mold and subsequent steps must also be considered. Further, the higher the mixing temperature of components (A) and (B), the faster the polymerization reaction. Thus, bubbles and optical defects tend to form in the molded article, limiting heating of component (A). Since the viscosity cannot be adequately lowered for this reason, the filtration rate of component (A) is slow. Additionally, although component (B) is of low viscosity, it must be mixed with component (A) in a prescribed ratio, so the level of mixed solution being discharged decreases.
Accordingly, it is conceivable to increase the volume (filtration surface area) of the filter used to filter component (A) to reduce the pressure loss exerted on the filter and ensure the targeted discharge level. However, since the mesh of the filter is fine and tends to clog, the filter maintenance interval becomes short. Further, after replacing the filter, bubbles mix into component (A) until the air within the filter is replaced by component (A). Thus, it is necessary to continuously pass component (A) through the filter until the bubbles disappear. When the filter area is large, a long period is required for the bubbles to disappear. There is thus a problem in that the RIM machine can only operate continuously for a short period.
Accordingly, the first object of the present invention is to provide a method of manufacturing optical members having good optical and mechanical characteristics, in particular, by removing, from component (A), not only foreign matter and debris, but also minute foreign matter tending to reduce the transmittance, optical properties, and mechanical strength of molded articles, without affecting the level of discharge of each component in an RIM machine.
Cast polymerization methods are known methods of molding plastic lenses. For example, Megane (published May 22, 1986 by Medical Aoi) discloses a manufacturing process for diethylene glycol bisallyl carbonate lenses (CR-39 lenses). This lens manufacturing process describes a casting mold in which a gap is maintained between the upper mold and lower mold of a glass master mold by means of a cylindrical gasket to form a cavity. A lens starting material liquid (referred to hereinafter as “monomer”) is cast into this cavity. Following casting, the mold is placed in an electric furnace and heated to conduct polymerization. The fully polymerized lens is then removed from the mold.
A monomer can be cast into a cavity through a casting inlet formed in a gasket. For example, Japanese Utility Model Publication (JIKKO) Heisei No. 6-39951 describes such a gasket. In the gasket disclosed therein, a ring-shaped protruding strip is provided in a circumferential direction along the surface of the inside wall of a cylindrical gasket main body. A portion of this protruding strip is cut away to form a notch part. A casting inlet is formed on the outer circumferential surface of the gasket main body adjacent to the notch part. The notch part and the casting inlet communicate through a casting hole formed in the gasket main body.
A cavity is formed in the gasket while maintaining an upper mold and a lower mold in a state of contact with the protruding strip.
This gasket is comprised of an elastic resin and integrally formed.
A mixed monomer solution is cast into the casting mold in which the gasket is employed by casting the monomer fluidly under the effect of its own weight through the casting inlet part with the gasket in a tilted position so that the casting inlet of the casting inlet part faces upward.
However, in the gasket disclosed in above-cited Japanese Utility Model Publication (JIKKO) Heisei No. 6-39951, there is a problem in that a penetrating hole is formed from the outer wall surface to the inner wall surface of the gasket main body as a casting hole, increasing the cost of manufacturing the injection mold. This is because the configuration of the injection mold generally becomes quite complex when using injection molding to integrally mold plastic molded articles having holes, for example.
Further, when casting a monomer from above as is the case with this gasket, bubbles are sometimes involved during casting, depending on the casting conditions and materials. When the viscosity of the monomer is low, there are few problems because the bubbles are relatively easily removed. However, when employing a high viscosity monomer or a monomer with a high initial polymerization rate, the removal of bubbles is difficult. A gasket of such configuration cannot be utilized when employing such starting materials.
U.S. Pat. No. 6,127,505 discloses a polymerization starting material of high viscosity and high initial polymerization rate. This starting material is comprised of an isocyanate terminal prepolymer having intramolecular urethane bonds and an aromatic diamine. The former prepolymer is of high molecular weight and high viscosity, and both are characterized by undergoing a rapid polymerization reaction immediately after being mixed with each other. Molded articles obtained by the method disclosed in U.S. Pat. No. 6,127,505 are of a high strength, comparable with that of polycarbonates.
To mold lenses using the materials disclosed in the above-cited U.S. Pat. No. 6,127,505, it is desirable to employ a reaction injection molding technique, rapidly mix the materials, and cast the materials into a casting mold immediately following mixing.
Of the materials disclosed in the publication, the aforementioned prepolymer has a high viscosity. Therefore, when bubbles are produced within the cavity, removal of the bubbles is difficult. That is, when casting the mixed solution from above the casting mold, air is involved in the mixed solution, forming bubbles. Due to the high viscosity, the bubbles that are formed tend not to rise upward, remaining within the molded article. Further, since the disclosed material begins to polymerize immediately following mixing at a rapid rate of polymerization, the viscosity tends to rise following casting, rendering the removal of bubbles all the more difficult.
Since immediately after mixing, the disclosed material begins polymerizing rapidly at a fast rate of polymerization, optical defects tend to form in the lens that is molded. That is, when the mixed solution is poured in from above the casting mold, flow occurred between the mixed solution cast in first and that cast in later is relatively active. Thus, marks like flows at that time and marks produced by uneven polymerization tend to be produced, sometimes becoming optical defects. Further, when the casting mold is subjected to vibration or shock shortly after casting the material, the mixed solution undergoing polymerization moves about within the cavity, with the resulting marks sometimes becoming optical defects.
For these reasons, there is a need for a method of molding in which bubbles and optical defects tend not to form in the molded article.
In molding methods employing the gasket disclosed in Japanese Utility Model Publication (JIKKO) Heisei No. 6-39951, the gasket is removed following polymerization from a casting mold that has been filled with starting material. In that process, it is necessary to sever, in the vicinity of the casting hole, the portion that has polymerized within the monomer casting inlet and remove it from the molded article. When employing a gasket such as that described in Japanese Utility Model Publication (JIKKO) Heisei No. 6-39951, the portion that has polymerized in the vicinity of the casting hole is usually broken by bending the casting inlet, and the gasket is removed.
However, the material disclosed in U.S. Pat. No. 6,127,505 is quite strong, as stated above, and thus not readily broken. Further, it is of high viscosity, so that when the casting hole is widened to facilitate casting, the strength of this portion becomes even greater and breaking becomes even more difficult. Thus, there is a need for a molding method permitting the ready removal of the gasket.
Japanese Examined Patent Publication (KOKOKU) Heisei No. 7-29320 discloses a molding method in which no air is involved within the casting mold in reaction injection molding. In this method, the casting inlet of the reaction solution mixture is positioned beneath the casting mold and an air discharge outlet is provided in the portion of the casting mold that is filled last. It is disclosed that introducing the reaction solution mixture from below permits the obtaining of a molded article free of bubbles that is attractive in appearance.
However, until now, there has been no gasket suited to the cast molding of plastic lenses by introducing from below a reactive monomer of relatively high viscosity such as that set forth above. Further, since the above monomer reacts rapidly and cures immediately, overflowing of the material from the casting mold to the exterior may create operational problems.
Accordingly, the second object of the present invention is to provide a molding method suited to the molding of plastic lenses from a mixed solution of an isocyanate terminal prepolymer component (A) that is the reaction product of an aliphatic diisocyanate having an intramolecular cyclic structure and a diol having an average molecular weight of 300 to 2,500, and an aromatic diamine component (B); and more particularly, to provide a method of manufacturing a polymeric molded article free of bubbles and optical defects achieved by improving the method of casting the mixed solution into the casting mold, and a method of readily removing a gasket following molding achieved by improving the molding method.
The third object of the present invention is to provide a gasket that is suited to the cast molding of plastic lenses by casting from below a monomer of viscosity and reactivity such as set forth above, and in which the casting inlet structure is a non-hole structure that is capable of being readily molded while tending not to cause starting material monomer to overflow to the exterior of the casting mold; a casting mold employing the aforementioned gasket; and a monomer casting jig suited to the casting of starting material monomer into the casting mold.